Alterations in the balance of tubulin glycylation and glutamylation in photoreceptors leads to retinal degeneration

Abstract

Tubulin is subject to a wide variety of posttranslational modifications, which as part of the tubulin code are involved in the regulation of microtubule functions. Glycylation has so far predominantly been found in motile cilia and flagella, and absence of this modification leads to ciliary disassembly. Here we demonstrate that the connecting cilia of photoreceptors, which are non-motile sensory cilia, are also dependent on glycylation. In contrast to many other tissues, only one glycylase, TTLL3, is expressed in retina. Ttll3−/− mice lack glycylation in photoreceptors, which results in shortening of connecting cilia and slow retinal degeneration. Moreover, absence of glycylation results in increased levels of tubulin glutamylation in photoreceptors, and inversely, hyperglutamylation in the pcd mouse abolishes glycylation. This suggests that both posttranslational modifications compete for modification sites, and that unbalancing the glutamylation/glycylation equilibrium on axonemes of connecting cilia, notwithstanding by which enzymatic mechanism, invariably leads to retinal degeneration.